How to Make Your Grid Pan Automatically

I got a question at an event a few weeks ago that sounded at first like it was going to implicate a rather complicated answer. In the end, it turned out to be not so bad.

The question was…

On the start screen when the user moves his mouse to the edge, the tiles pan automatically. How do I make the grid in my app work just like that?

And the answer - or rather an answer is here…

var timer = null; 
element.querySelector(".groupeditemslist").onmousemove = function (ev) { 
    clearInterval(timer); 
    timer = -1; 
    if (ev.screenX > (document.body.scrollWidth - 50)) 
        timer = setInterval(function () { listView.scrollPosition += 30; }, 1); 
    else if (ev.screenX < 50) 
        timer = setInterval(function () { listView.scrollPosition -= 30; }, 1); 
};

Let me unpack that for you.

When the user’s mouse is within the grid (that’s the .groupeditemslist), then check the event argument that our event function received and see what the location of the user’s mouse is on the screen (that’s screenX).

If it is withing 50 pixels of the right side of the page (that’s the document.body.scrollWidth) then set timer to a new interval that runs a simple function every 1 millisecond. The body of the function that runs is listView.scrollPosition += 30, which you have likely guessed scrolls (pans) to the right by 30 pixels. And the opposite should obviously happen if the user moves his mouse close to the left edge.

I wrote this rather quickly and have not tested it thoroughly. It appears to work well and does not have any significant performance impact (even though the interval is a single millisecond). That said - I would highly encourage you to tweak the numbers and see what works best for you. If you come up with a better combination, please leave a comment below.

I hope every is having as much fun with this Windows 8 development as I am!

Happy panning.